Electronic driver circuits are often called upon to drive electrical loads in a controlled manner in order to achieve a variable result. As one example, DC-DC switching converters (e.g., a buck converter, etc.) are often used to drive a light emitting diode (LED) load. The LED brightness is directly proportional to the current through the LED.
One type of converter sometimes used to drive an LED load is a current regulator that maintains a constant current output by using the output inductor as a current-regulating element. This regulator type eliminates the need for a large output capacitor as is generally required in voltage regulators, thereby reducing component cost and circuit board space requirements. However, one drawback of using the output inductor as a current-regulating element is that the inductor current cannot be turned on and off instantaneously.
A commonly used technique for adjusting LED brightness is the use of a pulse width modulation (PWM) signal. The LED current ramps up to a regulation target when the PWM signal is high and ramps down when the PWM signal goes low. Due to the finite current slope through the output inductor, the average current cannot be precisely controlled. The error gets larger as the PWM duty cycle gets lower. Therefore, another technique may be applied to achieve fast turn on and turn off of the LED current. One such, technique is to use a bypass switch in parallel with the LED string. The bypass switch acts as a current shunt to steer the inductor current away from the LED load such that the voltage across the LED string drops instantaneously from Vf (i.e., the forward voltage across the LED string) to nearly zero volts. The average inductor current remains unchanged, while the switch duty cycle is reduced to account for the lower output voltage.